hw/misc/grlib_ahb_apb_pnp: Fix 8-bit accesses

The Plug & Play region of the AHB/APB bridge can be accessed
by various word size, however the implementation is clearly
restricted to 32-bit:

  static uint64_t grlib_apb_pnp_read(void *opaque, hwaddr offset, unsigned size)
  {
      APBPnp *apb_pnp = GRLIB_APB_PNP(opaque);

      return apb_pnp->regs[offset >> 2];
  }

Set the MemoryRegionOps::impl min/max fields to 32-bit, so
memory.c::access_with_adjusted_size() can adjust when the
access is not 32-bit.

This is required to run RTEMS on leon3, the grlib scanning
functions do byte accesses.

Reported-by: Jiri Gaisler <jiri@gaisler.se>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-Id: <20191025110114.27091-3-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>